1. Field of the Invention
The present invention relates generally to data recording, and more particularly to archival recording of digital data in a readily retrievable compressed format.
2. Description of the Prior Art
There exist circumstances in which audio or low bandwidth data must be recorded together with the time when such recording occurred. Examples of various types of information that are so recorded include telephonic and radio conversations, radar data, facsimiles and modem communications. Such recordings are particularly necessary in police stations, air traffic control facilities, hospitals, prisons, brokerage houses and analogous environments. Systems capable of performing this function have been commercially available for years and are frequently referred to as recording loggers, or just loggers. Initially loggers were based on continuous, real-time analogue magnetic recording which can provide the functionality required for this particular application. However, such analog magnetic logger systems are physically large and require large amounts of space for storing audio tapes used to archive the audio recordings.
To overcome these drawbacks of prior analog loggers, digital loggers, that record onto digital audio tapes ("DATs"), are being used. Although both digital loggers and the DAT on which they record data are physically much smaller than the prior analog recording technology, they exhibit certain drawbacks. One difficulty initially experienced with digital loggers was that the DATs do not provide an efficient time related scheme which permits quickly and reliably retrieving a recording made at a particular time. Another disadvantage was that the prior digital loggers do not efficiently utilize the recording space available on the DATs. Still another disadvantage of prior digital loggers was that the DAT tape decks used with such loggers experience greater wear because they operated whether or not audio is being written.
U.S. Pat. No. 5,448,420 entitled "Method and a System for Storing Audio" that issued Sep. 5, 1995, ("the '420 patent"), discloses a multi input-channel apparatus and method that permits recording audio data on DATs for time-based retrieval. Furthermore, the digital logger disclosed there uses recording space on each DAT more efficiently, and activates the DAT tape deck only while recording. The digital logger first converts each input-channel's audio signal to digital data and then further compresses the data as from 64 k bits/sec/channel to 13 k bits/sec/channel. After the digital data has been compressed, it is stored into a buffer. Subsequently, the digital logger retrieves the data from the buffer and records it onto the DAT. In this way, the buffer matches the digitizing rate of audio processing to the DAT tape deck data transfer rate. Furthermore, data is recorded on the DAT only when the digital logger receives an audio signal, and not during intervals of silence.
To facilitate time-based retrieval of recorded data, the digital logger divides data recorded onto the DAT into a primary partition and secondary partition. The secondary partition is located at the beginning of the DAT. The secondary partition stores times for the beginning and end of data recorded on the DAT and the times and lengths of recording sessions that are read from a clock included in the digital logger. The secondary partition can contain information such as the media format, the manufacturers identification, the product identification by model number of the tape deck that formatted the DAT, the drive vendor, the logic unit type, the logic unit software version, and other similar information. In addition, the secondary partition stores tables to be described that are used in retrieving particular recorded data.
The primary partition includes a plurality of a filemarks. Each filemark is followed by a header and at least one data group, with each data group containing a plurality of audio blocks. Each audio block records audio from an input-channel to the digital logger. Wile the digital logger receives no audio on any input-channel, no header or group is generated and only a plurality of successive filemarks are recorded on the DAT. Each filemark represents a unit of time, as for example six (6) seconds, and each group has an allotted time, as for example approximately one and two-tenths (1.2) seconds. There are, for example, five allotted time periods between filemarks. Space allocation for a group occurs when the digital logger receives an audio signal on the corresponding input-channel during the allotted time interval. Even though each group may not be recorded, each filemark recorded on the DAT still represents six seconds. Each group has a specified number of audio blocks, as for example 32, each audio block representing 1.2 seconds. Thus, if any audio block receives audio from its respective input-channel during the allotted 1.2 seconds, the data group records 1.2 seconds of the audio signal.
Each header recorded on the DAT contains a group map table which specifies a correspondence between the input-channel number and the group number and indicates the presence or absence of audio recorded from particular input-channels. During a recording session, each group map table is written into its respective header. After each recording session, that sessions's group map table data are also written into an accumulated table in the secondary partition located at the beginning of the DAT. Using the digital logger's clock, a recording session's start time is written into the secondary partition as well as the recording session's end time. In this way, the secondary partition accumulates a table representing the group map tables in all headers recorded on the DAT. When retrieving an audio recording from the DAT, the digital logger uses the group map table data recorded in the secondary partition to locate audio blocks recorded from a particular input-channel into a series of data groups.
Although the digital loggers disclosed in the '420 patent has advantages over prior analog loggers, it still has shortcomings in terms of networking expandability and voice capacity. U.S. Pat. No. 5,819,005 entitled "Modular Digital Recording Logger" that issued Oct. 6, 1998, ("the '005 patent"), discloses a digital logger having a basic unit that includes four (4) primary components, an audio card having a plurality of input-channels each of which concurrently communicates with several independent sources such as telephones, a main card that processes audio, a host computer that controls the digital logger's overall operation, and a memory.
In communicating with the audio sources, each audio card receives analog signals and converts them to digital data and forwards the digital data via a time division multiplexed (TDM) bus to the main card. The main card communicates with audio cards through the TDM bus to monitor the status of the audio cards and thereby determine which needs service. The main card also packages data retrieved from the audio cards, executes speech compression and expansion, and performs VOX and other functions. The main card also connects to an Industry Standard Architecture ("ISA") bus as are a micro-processor a LAN adapter and a SCSI adaptor. A computer program executed by the micro-processor supervises and coordinates the activities of the other digital logger components. For recording the compressed digitized audio data, the SCSI adaptor also communicates with at least one DAT tape deck and at least one hard disc drive. Recording audio data stored onto the hard disk permits replaying a desired portion of such data while the digital logger continues recording data both onto the DAT and onto the hard disk.
The digital logger disclosed in the '005 patent is modular so its capacity can be expanded as required readily at a minimum cost and software can be modified conveniently as desired. In addition, the LAN adapter allows each digital logger described in the '005 patent to be part of a networked system that includes other digital loggers and workstations. Workstations connected to the LAN are similar to the digital logger except they omit the SCSI adaptor, DAT tape deck and hard disc drive. As described in the '005 patent, indicates that workstations may only receive compressed digitized audio data from the digital logger via the LAN for replaying at the workstation.
However, despite all of the effort expended thus far in attempting to configure digital loggers to record effectively and conveniently on DAT, economic impediments and operational difficulties still exist. For example, the price for DAT tape decks is two (2) to three (3) times greater than that for archival optical recording technologies such as a write-once compact disk ("CD-R") recorders. Furthermore, when used in digital loggers DAT tape decks exhibit poor reliability, e.g. a 2000 hour mean time between failure ("MTBF"), because of the mechanical complexity required for helical scan magnetic recording used for DAT.
Furthermore, when used in a digital logger the DAT tape deck must either be left running all the time, or be frequently turned on and off. Either of these operating modes is to be contrasted with the operating mode for which DAT tape decks are designed in which the DAT tape deck is turned on and off infrequently, and remains turned on for a comparatively long interval of time, e.g. tens of minutes. Because DAT tape decks are not designed for either of the operating modes which a digital logger must employ, such tape decks exhibit poor reliability when used for that purposes.
In addition to characteristics of DAT tape decks which make them unattractive for audio logging, the formats used for recording digital logger data on DAT such as that as described in the '420 patent and retrieving data so recorded is inefficient and time consuming. While recording data on DAT, the digital logger must accumulate and maintain the data to be subsequently recorded in the secondary partition located at the beginning of the DAT. Periodically, the digital logger must rewind the DAT to the beginning, record the data in the secondary partition, and then return to the end of the recorded data to resume recording.
After data has been recorded onto the DAT, isolating and recovering a particular telephone conversation from the DAT is an arduous process. Retrieving a specific telephone conversation from DAT requires first rewinding the tape to read the secondary partition, and then searching down the tape to listen to audio blocks for a particular input-channel which may or may not contain a desired conversation. If the wrong audio block and or channel has been chosen, listening to other audio blocks for the same or another input-channel requires again rewind the DAT to re-read the secondary partition, and then again searching down the tape to listen to other audio blocks.
In addition to the preceding problems associated with using DAT for digital loggers, under certain circumstances the recording format for telephone conversations logged on DAT using the DAT recording technique described in the '420 patent may impede law enforcement. If in the course of a judicial proceeding a law enforcement agency must produce as evidence a copy of a conversation recorded on DAT using that technique, then the digital data recorded for the input-channel in question will necessarily be intermixed with all other concurrently logged conversations from other input-channels of the digital logger. The intermixing of two or more conversations on a single DAT tape that must be disclosed as part of a judicial proceeding creates a possibility that other concurrently recorded conversations could be exploited to unwarrantedly impede the judicial process.
Furthermore, as has been known for many years, magnetic tape is not considered an archival recording media. Over time, magnetic tape recording media degrades and the information recorded there becomes un-recoverable. Consequently, archival data bases that have been recorded on magnetic tape media must be periodically refreshed by copying data stored on older media onto newer media. For example, commercial manufacturers project only a seven (7) year life for DATs. Conversely, it is known that archival recording media, having a useful life of one-hundred ("100") years, exists for optical recording technologies such as microfilm. Manufacturers of media for newer optical recording technologies such as a CD-R, CD-RW, write-once digital video disk ("DVD-R"), and rewritable digital video disk ("DVD-RW") presently estimate the useful life for such recording media at thirty (30) to thirty-five (35) years to one-hundred (100). However, several technological impediments exist to directly using CD-R, CD-RW, DVD-R and CD-RW technologies in the conventional way for analog data recording applications such as that required for a digital logger.
First, the conventional consumer CD audio recording format can presently preserve only seventy-four (74) minutes of stereo per CD-R or CD-RW. Conversely, audio logging requires a minimum of several input-channel-hours of recording time per day, which makes direct use of conventional CD audio recorders impractical for that application. Furthermore, consumer quality audio CD recorders have a MTBF of approximately one-thousand ("1,000") hours. Consequently, even if the conventional audio CD recording format were somehow inherently capable of recording several input-channel-hours of audio recording per day, the failure of such a recorder approximately every forty (40) days would bar using that technology in digital loggers.
While there exists a "multi-session" recording format for CD-R technology that can be used to record compressed digital data representing an analog signal, each successive recording session consumes approximately 20 Mega-bytes ("MBs") of storage space on the CD-R media. Since CD-R and CD-RW media provide approximately 650 MBs of storage, space occupied on CD-R media for a sequence of approximately thirty-two (32) multi-session recordings completely fills a single CD.